Process for vacuum degassing with electromagnetic stirring



Jan. 18, 1966 c. ERICSSON 3,230,073

PROCESS FOR VACUUM DEGASSING WITH ELECTROMAGNETIC STIRRING Filed July20, 1962 INVENTOR.

Curl, Erz'csson United States Patent O 3,230,073 PROCESS FOR VACUUMDEGASSING WITH ELECTROMAGNETIC STIRRING Curt Ericsson, Surahammar,Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras,Sweden, a Swedish corporation Filed July 20, 1962, Ser. No. 211,380 2Claims. (Cl. 75-10) This application is a continuation-in-part of ourcopending application Serial No. 734,866, filed on May 13, 1958 and nowabandoned, and entitlted Improvements in the Manufacture of Metal andMetal Alloys.

In the manufacture of metal and metal alloys, the molten metal isusually poured from the melting furnace into a ladle from which it isthen transferred to the moulds that form the ingots. In order to obtainingots that are free from gas inclusions, it has been proposed to placethe ladle in a vacuum chamber and thereafter, when a certain time haselapsed, possibly also under vacuum, to pour the metal into the mouldsforming the ingots.

It has now been found that this kind of vacuum treatment gives anunsatisfactory result, in spite of long evacuation, because only the toplayer in the metal bath in the ladle will be freed from gas in asatisfactory way. This fact can be observed when pouring the melt intothe moulds, the major part of the melt giving off a large amount of gas.The explanation of this is that the hydrostatic pressure in the moltenmetal in the ladle is too high for the gases dissolved in the metal tobe given off while the metal is still in the ladle.

The present invention concerns equipment for and a method of forcing thegases to leave the melt while the metal is still in the ladle, insteadof remaining until the pouring. According to the invention, this isachieved by stirring the melt electro-magnetically while it is still inthe ladle, so that all parts of the melt will gradually come up to thesurface of the melt thereby enabling the melt to give off its gases.According to the invention this stirring of the metal melt is achievedby means of stationary electromagnetic stirring coils fed by a suitablelow-frequency current. By low-frequency is meant here and in thefollowing, 0.5-l Hz. The stirring is to be accomplished by multi-phasestirrers, the number of phases being 2-3. In one arrangement included inthe invention, the magnetic field necessary for stirring the melt isproduced by helical coils supplied immediately outside of the ladle,these coils being placed on a number of long steel cores, verticallyarranged along the outer wall of the ladle. The coils can also be madeas pancake coils arranged on the side of the steel core that is towardsthe ladle wall such cores would then be referred to as yokes.

The stirring magnetic field can also be created by means of cylindricalcoils surrounding the ladle and concentric with it. The coils and thecores or yokes can either be solidly connected to the ladle or placed ina stationary position in the chamber into which the ladle is introducedfor vacuum treatment. The coils and cores can also be placed outside thevacuum chamber, for example so that the ladle itself becomes the vacuumchamber.

The moulds into which the molten metal is to be poured possibly can bearranged either inside the vacuum chamber or outside.

The attached drawing shows an example of an arrangement for obtainingthe described de-gassing. FIG. 1 shows a vertical section and FIG. 2 ahorizontal section of the arrangement.

The ladle shown in the example consists of a cylindrical vessel withlining 1 and steel wall 2 and is placed inside a 3,230,073 Patented Jan.18, 1966 vacuum chamber suitably consisting of a bell 3 and a bottom 4,attached to the bell by means of a vacuum-type seal and having aconnection pipe leading to a vacuum pumpin arrangement. 6 is a stopperarrangement with operating bath for controlling the pouring of the meltwhen filling the moulds. Multi-phase, preferably twophaseelectro-magnetic stirrers are arranged outside the wall of the ladle,each of them consisting of coils fed with low-frequency current.Electro-magnetic stirrers are arranged outside the wall of the ladle,each stirrer in the example having two windings, each of them consistingof coils 7, fed with low-frequency current. The coils are arranged on astraight iron core 8. It is not necessary to have two stirrers, but inthe case of there being two stirrers, they can be either series orparallel connected. In the example shown, the stirrers are fixed ontothe inner side of the vacuum chamber but it is, as mentioned before,possible to fix them onto the ladle or to support them from the bottomof the chamber, on which bottom the ladle in the example is also placed.

The pressure in the bell 3 above the surface of the melt is afterevacuation to be 0.1-50 Torrs, preferably above 20 Torrs. The number ofphases is two or three, preferably two.

Instead of the coils described above and in the drawing, it is alsopossible to arrange the coils concentrically around the ladle.

In view of the heavy current losses in the ladle wall, it is convenientto replace the reinforcement girdles of the ladle by vertical steelbars.

I claim as my invention:

1. Method for treating a molten metal which comprises melting the metal,placing the molten metal in a container having a steel shell, placingthe container in a vacuum chamber in the field of a stationaryelectromagnetic multiphase stirring means positioned outside and at theside of the container, evacuating the chamber, and subjecting the moltenmetal while within the vacuum container to a stirring action by feedingto the stirring means current at a frequency of between about 0.5 and 10cycles per second, said stirrer producing vertical movements in themolten metal from parts close to the bottom of the container to thesurface of the melt.

2. A method as claimed in claim 1 in which the pressure in the chamberis between 0.1 and 50 Torr.

References Cited by the Examiner UNITED STATES PATENTS 428,552 5/ 1890Colby. 1,277,523 9/1918 Yensen -49 2,513,082 6/ 1950 Dreyfus. 2,675,4144/ 1954 Capita. 2,686,823 8/1954 Jones 1326 FOREIGN PATENTS 3 39,579 12/1930 Great Britain. 702,225 l/ 1954 Great Britain.

OTHER REFERENCES Byrne, J. W.: Worlds Largest Vacuum Induction Furnace,Metal Progress, April 1960, pages 83-86.

Metal Industry, September 20, 1957, pages 234-235.

Tix et al.: Vacuum Treatment of Steel by the Bochumer Verein StreamDegassing Process, Journal of the Iron and Steel Institute, March 1959,pages 260-265.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

1. METHOD FOR TREATING A MOLTEN METAL WHICH COMPRISES MELTING THE METAL,PLACING THE MOLTEN METAL IN A CONTAINER HAVING A STEEL SHELL, PLACINGTHE CONTAINER IN A VACUUM CHAMBER IN THE FIELD OF A STATIONARYELECTROMAGNETIC MULTIPHASE STIRRING MEANS POSITIONED OUTSIDE AND AT THESIDE OF THE CONTAINER, EVACUATING THE CHAMBER, AND SUBJECTING THE MOLTENMETAL WHILE WITHIN THE VACUUM CONTAINER TO A STIRRING ACTION BY FEEDINGTO THE STIRRING MEANS CURRENT AT A FREQUENCY OF BETWEEN ABOUT 0.5 TO 10CYCLES PERR SECOND, SAID STIRRER PRODUCING VERTICAL MOVEMENTS IN THEMOLTEN METAL FROM PARTS CLOSE TO THE BOTTOM OF THE CONTAINER TO THESURFACE OF THE MELT.